The present invention relates to an X-ray imaging system. In particular, the invention relates to a system of dental X-ray imaging using a complementary metal oxide semiconductor active column sensor adapted for X-ray imaging.
An active pixel is a semiconductor device capable of converting an optical image into an electronic signal. Active pixels can be arranged in a matrix and utilized to generate video signals for video cameras, still photography, or anywhere incident radiation needs to be quantified. When incident radiation interacts with a photosite, charge carriers are liberated and can be collected for sensing. The number of carriers collected in a photosite represents the amount of incident light impinging on the site in a given time period.
There are two basic devices, i.e., photodiodes and photogates, with many variants, employed to collect and sense, charge carriers in a photosite. Variants of photodiodes include Pinned, P-I-N, Metal-Semiconductor, Heterojunction, and Avalanche. Photogate structures include: Charge Coupled Devices (CCD), Charge Injection Devices (CID), and variants that include virtual phase, buried channel and other variations that utilize selective dopants which are used to control charge collection and transfer underneath and between the photogate(s) and the sense node.
Solid state imagers heretofore used have been dominated by CCDs because of their low noise as compared to photodiodes and CIDs. However, the low noise of the CCD requires the imager to be read in a fixed format and once the charge is read it is destroyed.
Solid state imaging devices have developed in parallel with CMOS technology and as a result proprietary processes have been developed to maximize imager performance characteristics and wafer yield. Coupling the collected photon charge from the pixel to the periphery amplifier typically requires proprietary processing steps not compatible with industry standard CMOS (complementary metal oxide semiconductor) or BiCMOS (bipolar CMOS) processes. There has been a movement to transfer the proprietary processes to an industry standard CMOS process which provides advantages such as competitive wafer processing pricing, and on chip timing, control and processing electronics.
A CMOS compatible, CID imager has been fabricated. The imager could either be operated as a random access CID, or all the columns could be summed together and operated as a linear active pixel sensor. The imager has a preamplifier and correlated double sampling (CDS) circuit per column. The CDS circuit stores the initial value of each pixel before each frame is captured, which indicates the offset compared to true black, i.e., zero incident light. After the frame is captured, each pixel""s value is adjusted up or down based on that initial value.
U.S. Pat. No. 5,471,515 describes area arrays utilizing active pixel sensors in which a photodiode or photogate is coupled to an output source follower amplifier which in turn drives a CDS circuit. Two outputs of the CDS cell then drive two more source followers circuits that in turn are fed into a differential amplifier. The source follower circuits typically have gains less than unity that vary from one source follower to another. The source follower gain variation is due to variations of FET thresholds, and results in a pixel to pixel gain mismatch. Also, the active pixel sensors suffer gain variations due to the CDS circuit per column, when the CDS employs a source follower pair to drive its output. The resulting CDS signal and its corresponding offset can have different gains that are not correctable by the differential amplifier. Also, the source follower configuration of active pixels does not allow for binning of pixels, i.e., summation of two or more pixel signals at once.
CMOS image sensors now also are available. The CMOS image sensor is an integrated circuit (IC) that detects and converts incident light (photons) into electronic charge (electrons) through a photoconversion process. The sensor includes an array of photodiodes that can detect light in the visible spectrum. CMOS transistors in each pixel select, amplify and transfer the photodiode signals. A CMOS imager or imaging system may include both the sensor and supporting circuitry for further amplifying and processing the detected image. CMOS imagers offer several significant advantages over CCD imagers, including lower overall cost, lower power requirements and a higher level of integration that can reduce the size of the imaging system. In addition, CMOS imagers are relatively easy to manufacture in standard CMOS wafer fabrication facilities. Most CMOS imagers today use Active Pixel Sensor (APS) technology, which utilizes an amplifier for each pixel. Each of those amplifiers requires at least three field effect transistors (FET) to implement. Due to process variations during the manufacture of these amplifiers, the actual gain and offset of each amplifier is slightly different from those of the other amplifiers. As a result, APS imagers suffer from high fixed pattern noise (FPN) problems. The resulting video can appear as if viewed through a dirty, scratchy window.
Some APS systems counteract the gain and offset issues by creating tables of multiplier and offset values to correct the incoming video. As long as the temperature remains relatively constant, these tables can be used with little or no modification. However, the tables add to the complexity of the system.
A solution to the variation in gain from one amplifier to another is to implement a unity gain amplifier (UGA) for every pixel. Each UGA requires, however, the use of at least six FETs, which increases the complexity of the product significantly. In addition, each FET decreases the active area of the corresponding pixel. With six or more FETs per pixel, the active area is greatly reduced. Further, the added complexity raises the cost of the product, which eliminates one of the major advantages that CMOS technology is intended to provide.
Electronic image sensors, such as CCD or CMOS pixel sensors, have been adapted to be X-ray sensitive elements in dental and medical applications. The digital X-ray sensor is used to detect and record X-ray images which typically are downloaded to a personal computer via a cable. Examples of use of CCD-type and other X-ray image sensors in dental and/or medical environments are described in U.S. Pat. Nos. 5,671,738 and 5,744,806, which are incorporated herein by reference. An X-ray detector that comprises a plurality of CMOS active pixel sensors is described in U.S. Pat. Nos. 5,912,942 and 6,069,935.
As discussed above, CCD sensors and CMOS active pixel sensors, however, have their disadvantages.
The present invention provides an X-ray imaging system comprising, in accordance with one embodiment, an X-ray image sensor including a plurality of CMOS active column sensor elements, and image capture controller communicating with the X-ray image sensor to read out pixel values from selected ones of the plurality of CMOS active column sensor elements of the X-ray image sensor. The image capture controller randomly may select reference pixels from the plurality of CMOS active column sensor elements, and compare a signal of the reference pixels to a predetermined level to determine a start of X-ray exposure. The image capture controller also may randomly select second reference pixels from the plurality of CMOS active column sensor elements, and monitor the second reference pixels to determine and end of X-ray exposure.
The X-ray imaging system also may include a wireless interface for coupling the X-ray image sensor to the image capture controller.
The X-ray imaging system may be used as a dental X-ray imaging system.
The present invention also provide an intraoral X-ray image sensor comprising, in accordance with one embodiment, a scintillator that converts an X-ray energy image into a visible-light image, and a sensor array including a plurality of CMOS active column sensors, said sensor array converting the visible-light image into an electrical signal.